Hi Bogdan, You can also get the information from the link of the Wilcox.test function page.
“By default (if exact is not specified), an exact p-value is computed if the samples contain less than 50 finite values and there are no ties. Otherwise, a normal approximation is used.” For more: https://stat.ethz.ch/R-manual/R-devel/library/stats/html/wilcox.test.html Hope this helps! Best, VD On Thu, Mar 18, 2021 at 10:36 PM Bogdan Tanasa <tan...@gmail.com> wrote: > Dear Peter, thanks a lot. yes, we can see a very precise p-value, and that > was the request from the journal. > > if I may ask another question please : what is the meaning of "exact=TRUE" > or "exact=FALSE" in wilcox.test ? > > i can see that the "numerically precise" p-values are different. thanks a > lot ! > > tst = wilcox.test(rnorm(100), rnorm(100, 2), exact=TRUE) > tst$p.value > [1] 8.535524e-25 > > tst = wilcox.test(rnorm(100), rnorm(100, 2), exact=FALSE) > tst$p.value > [1] 3.448211e-25 > > On Thu, Mar 18, 2021 at 10:15 PM Peter Langfelder < > peter.langfel...@gmail.com> wrote: > > > I thinnk the answer is much simpler. The print method for hypothesis > > tests (class htest) truncates the p-values. In the above example, > > instead of using > > > > wilcox.test(rnorm(100), rnorm(100, 2), exact=TRUE) > > > > and copying the output, just print the p-value: > > > > tst = wilcox.test(rnorm(100), rnorm(100, 2), exact=TRUE) > > tst$p.value > > > > [1] 2.988368e-32 > > > > > > I think this value is what the journal asks for. > > > > HTH, > > > > Peter > > > > On Thu, Mar 18, 2021 at 10:05 PM Spencer Graves > > <spencer.gra...@effectivedefense.org> wrote: > > > > > > I would push back on that from two perspectives: > > > > > > > > > 1. I would study exactly what the journal said very > > > carefully. If they mandated "wilcox.test", that function has an > > > argument called "exact". If that's what they are asking, then using > > > that argument gives the exact p-value, e.g.: > > > > > > > > > > wilcox.test(rnorm(100), rnorm(100, 2), exact=TRUE) > > > > > > Wilcoxon rank sum exact test > > > > > > data: rnorm(100) and rnorm(100, 2) > > > W = 691, p-value < 2.2e-16 > > > > > > > > > 2. If that's NOT what they are asking, then I'm not > > > convinced what they are asking makes sense: There is is no such thing > > > as an "exact p value" except to the extent that certain assumptions > > > hold, and all models are wrong (but some are useful), as George Box > > > famously said years ago.[1] Truth only exists in mathematics, and > > > that's because it's a fiction to start with ;-) > > > > > > > > > Hope this helps. > > > Spencer Graves > > > > > > > > > [1] > > > https://en.wikipedia.org/wiki/All_models_are_wrong > > > > > > > > > On 2021-3-18 11:12 PM, Bogdan Tanasa wrote: > > > > < > > https://meta.stackexchange.com/questions/362285/about-a-p-value-2-2e-16> > > > > Dear all, > > > > > > > > i would appreciate having your advice on the following please : > > > > > > > > in R, the wilcox.test() provides "a p-value < 2.2e-16", when we > compare > > > > sets of 1000 genes expression (in the genomics field). > > > > > > > > however, the journal asks us to provide the exact p value ... > > > > > > > > would it be legitimate to write : "p-value = 0" ? thanks a lot, > > > > > > > > -- bogdan > > > > > > > > [[alternative HTML version deleted]] > > > > > > > > ______________________________________________ > > > > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > > > > https://stat.ethz.ch/mailman/listinfo/r-help > > > > PLEASE do read the posting guide > > http://www.R-project.org/posting-guide.html > > > > and provide commented, minimal, self-contained, reproducible code. > > > > > > ______________________________________________ > > > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > > > https://stat.ethz.ch/mailman/listinfo/r-help > > > PLEASE do read the posting guide > > http://www.R-project.org/posting-guide.html > > > and provide commented, minimal, self-contained, reproducible code. > > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- ---------------------------------------------------------- Vivek Das, PhD [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
